Electroplating of metals using pulsed reverse current for control of hydrogen evolution

ABSTRACT

Excessive evolution of hydrogen in electrolytic deposition of metals on a cathode substrate can be controlled by using a pulsed reverse current. Reverse current pulses interposed between the forward current pulses consume at least some of the nascent hydrogen and prevent the local pH at the cathode surface from becoming excessively alkaline. Control of hydroxide ion concentration by pulsed reverse current alleviates problems caused by reaction of metal-bearing-ions with hydroxide ions generated near the cathode by evolution of hydrogen. The method is useful in depositing functional chromium coatings on electrically conductive substrates from plating baths comprising aqueous solutions of trivalent chromium salts. In such a method the current comprises forward pulses having a duty cycle of from about 50% to about 90% and reverse pulses having a duty cycle of from about 5% to about 30%, and a frequency of from about 5 Hz to about 700 Hz.

ORIGIN OF THE INVENTION

[0001] The experimental work leading to this invention was funded inpart by the U.S. Government Environmental Protection Agency Contract No.68D40033.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to methods of electroplating metals onto asubstrate and more particularly to electrodeposition of metals usingpulsed reverse current for controlling evolution of hydrogen.

[0004] 2. Brief Description of the Prior Art

[0005] Electrodeposition of metal coatings onto a substrate is a processthat is widely used in modern industry. Such electrodeposited coatingsare usually applied to metallic substrates and are generally intended toprovide enhanced surface properties to the base metal. For example,metal coating layers are applied to a base metal to prevent corrosion,enhance surface hardness, provide a smooth surface having a relativelylow coefficient of friction, and the like.

[0006] Metal coatings are ordinarily deposited by providing a platingbath which is an aqueous solution of metal-bearing ions, typicallysimple ions such as Cr³⁺, Zn²⁺, Au³⁺, Cd²⁺, which are generally presentas aquo complexes, or complex ion containing the metal, such as Cr₂O₇^(—). The metals are present in these ions in a positive oxidationstate, e.g. Cr(III) or Cr(VI). The substrate to be plated is immersed inthe plating bath and made the cathode of an electrolytic cell. The metalions are reduced at the surface of the cathode and deposited thereon asa layer of metal.

[0007] The actual mechanism of the cathodic reduction and deposition canbe complex, and is not well understood for many practical systems.Because the plating baths are aqueous solutions, electrolysis of waterwith evolution of hydrogen is usually a competing reaction at thecathode. The hydrogen formed may itself present problems, such ashydrogen embrittlement of the deposited metal coating or interferencewith the metal deposition caused by bubbles. The removal of hydrogen,with concomitant formation of OH⁻ ions, also increases the pH of theplating solution adjacent to the surface of the cathode. A high pH inthe plating layer may also produce problems such as formation ofinsoluble metal hydroxide layers on the cathode surface which alsointerfere with the transportation of the metal-bearing ions and thedeposition of metal atoms on the surface.

[0008] In order to prevent the problems associated with hydrogenevolution at cathodic plating surfaces, the plating industry has adopteda number of expedients. The metal-bearing ions that are commonly used inindustrial electroplating have been found to minimize the effects ofhydrogen evolution. For example, chromium is conventionally plated froma chromate bath in which the metal is present in the hexavalent state(Cr(VI)), and gold is generally plated as the cyanide salt.

[0009] Moreover, certain metals present problems in depositing layershaving satisfactory properties such as uniformity, luster and hardness,especially at useful plating rates. To overcome these problems theplating industry has developed plating baths that contain variousadditives that enhance the rate or ease of electrodeposition and theproperties of the coatings.

[0010] Some of these expedients have resulted in the use of platingbaths that are hazardous to use and difficult to dispose of byenvironmentally benign procedures. For example, it is has been foundthat the best results in gold electroplating are achieved using cyanidesolutions which are evidently hazardous to use and difficult toremediate for disposal. Similarly, it is customary to plate chromiumlayers, particularly functional chromium layers, from solutions whereinthe chromium is in the hexavalent state, typically as chromate ordichromate ions, a hazardous and carcinogenic form of the metal.

[0011] The workplace and environmental problems experienced withchromium plating are especially pressing because of the very extensiveuse of electroplated chromium coatings in industry.

[0012] Chromium coatings on a base metal are widely used in automotive,aerospace and other industries to provide a finished article withsurface properties that are not inherent in the base metal itself or areattainable only by using expensive alloys. Such coatings are usuallydeposited on the base metal by electroplating. Two types of chromiumcoatings are used, conventionally identified as functional coatings anddecorative coatings. Functional coatings consist of a relatively thicklayer of chromium (typically 1.3 to 760 micrometers thick) to provide asurface with functional properties such as hardness, corrosionresistance, wear resistance, and low coefficient of friction. Suchfunctional coatings are used on automotive strut and shock absorberrods, hydraulic cylinders, crankshafts, and industrial rolls. Carbonsteel, cast iron, stainless steel, copper, aluminum, and zinc aresubstrates commonly used with functional chromium layers.

[0013] Decorative coatings consist of a thin layer of chromium(typically 0.003 to 2.5 micrometers, plated over a nickel layer) toprovide a bright surface with wear and tarnish resistance. Such coatingsare used on automobile bumpers and trim, bath fixtures, and smallappliances.

[0014] Chromium is generally plated from an aqueous solution containingsoluble chromium species wherein the chromium is in the hexavalent state(Cr(VI)). Such a hexavalent chromium plating bath is a chromic acidsolution containing various chromate ions, dichromate ions, dichromateand trichromate. Sulfuric acid has been recognized as an essentialingredient of Cr(VI) plating baths.

[0015] Although plating from Cr(VI) baths has been the dominantcommercial procedure for a long time, the process has certaindisadvantages. A Cr(VI) plating bath is typically operated at atemperature significantly above room temperature and produces a mist ofchromic acid. Consequently, measures to protect the workers fromexposure to the toxic fumes are required by safety rules and by law.Exhaust/scrubber systems must be installed to keep the chromiumconcentration in the workplace atmosphere no greater than the prescribedlimit of 0.01 mg/m³. The amount of chromium that can be emitted to theair and water of the environment is also strictly regulated by federaland local law. Some deposition of decorative chromium plating is donefrom trivalent chromium (Cr(III)) baths. Plating from Cr(III) bathsinstead of Cr(VI) baths has several environmental advantages.

[0016] 1.) Cr(III) is non-toxic, non-hazardous and is not an oxidizingagent. Therefore, meeting air quality regulations is easier and workingconditions are greatly improved. The exposure limit for Cr(III) is anorder of magnitude higher than for Cr(VI).

[0017] 2.) Waste disposal costs for Cr(III) plating are significantlyless than for Cr(VI) plating. Hydroxide sludge generation is reduced tento twenty times because a Cr(III) bath generally contains only about4-20 g/liter of chromium, as opposed to 150-300 g/liter for a Cr(VI)bath.

[0018] 3.) A Cr(III) bath may be used without additives, which permitsthe rinse water from the plating operation to be recycled readily.

[0019] Plating from a Cr(III) bath also has certain technicaladvantages.

[0020] 1.) Current interruptions have little effect on the plating.

[0021] 2.) A Cr(III) bath is not affected by drag-in of chloride andsulfate from any previous nickel plating operations. In contract,chloride and sulfate drag-in upset the catalyst balance in Cr(VI) baths.

[0022] 3.) The throwing power of a Cr(III) bath, i.e., is ability toprovide a uniform coating of chromium to recesses on the surface of theobject to be plated, is greater than that of a Cr(VII) bath.

[0023] The effect of the plating bath chemistry, i.e., the compositionof the solution, on the plating thickness, brightness, hardness, andcorrosion resistance of chromium layers deposited from a Cr(III) bathhave been studied by several authors. The effect of the waveform of theplating current on the structure of the chromium deposit, and itsdistribution, brightness and hardness have also been studied. CommercialCr(III) baths are available that incorporate certain proprietary organiccompounds as additives in order to provide baths for decorative chromiumcoating applications. However, the concentration of the additives isdifficult to control because they are present in very small amounts.Furthermore, the additives react and break down with the passage of timeto form contaminants. Consequently, the used Cr(III) bath and the rinsewater from such plating operations cannot be replenished and/or recycledbecause the concentration of the contaminants would build up tounsatisfactory levels. Finally, decorative plating from a Cr(III) bathsuffers from low current efficiency.

[0024] Currently, functional chromium coating from a Cr(III) bath is notcommercially practical because it is difficult to plate thick chromiumcoatings with appropriate properties. Furthermore, the low currentefficiency and low plating rate of Cr(III) baths lead to unfavorableeconomics.

[0025] Attempts to plate gold from non-cyanide solutions have alsoexperienced difficulties. Gold plating baths that do not employ cyanideusually contain sulfite. Gold is deposited from the sulfite complexaccording to the equation

M₃Au(SO₃)₂+H₂O+e ⁻→Au+M₂SO₃+MHSO₃+OH⁻

[0026] where M is an alkali metal or ammonium ion. The sulfite ion isitself in equilibrium with sulfur dioxide according to the equation

SO₃ ²⁻+H₂O→SO₂(g)+20H⁻

[0027] Because this reaction forms hydroxyl ions, the equilibrium ispH-dependent, and the sulfite ion is ordinarily stable only at alkalinepH. Because the plating reaction generates OH⁻, the pH near the platedsurface (cathode) is usually very high. At alkaline pH, sulfite ionsaccumulate as gold is consumed and the specific gravity of the solutiontends to increase continuously as the bath is operated. This isundesirable for high speed operation, or for applications requiringselectivity. It would clearly be desirable to operate a sulfite goldplating bath under conditions such that sulfur dioxide is volatilized atapproximately the same rate at which gold is plated out. In such a case,the process tends to be self-regulating, and would operate in a fashionanalogous to that of the cyanide gold plating solutions.

[0028] Another electroplating application wherein it is desirable tocontrol hydrogen evolution and the local pH in the region of the cathodeis the developing attempts to substitute zinc-nickel or zinc-tin platingfor anti-corrosion coatings of cadmium in order to eliminate the use ofthat toxic metal. Zinc-based alloys, such as Zn—Ni and Zn—Sn are strongcandidates to replace cadmium. However the current electroplatingprocess for zinc alloy coatings suffers from two main difficulties:

[0029] 1) It requires a hydrogen-relief bake post-treatment to eliminatehydrogen embrittlement.

[0030] 2) It is difficult to control the composition of the alloy asdeposited.

[0031] Zinc alloy plating suffers from what is known in the platingindustry as anomalous deposition. The anomaly involved is the tendencyin such systems for the less noble metal to be deposited preferentially.In the case of the zinc-based alloys the result is a coating thatcontains more zinc and less nickel or tin than desired. According to oneof the leading proposed mechanisms the problem is caused by theformation of a zinc hydroxide film within the double layer adjacent tothe cathode surface that inhibits the electrodeposition of the morenoble metal. Attempts have been made to correct the anomalous depositionby adjusting the composition of the plating bath, but the results havenot permitted zinc alloys to replace cadmium extensively.

[0032] Still another industrial use of hexavalent chromium compounds incoating applications is the formation of anti-corrosive chromateconversion coatings on aluminum. A recent process developed forreplacing chromium in such coatings is the formation of acerium-molybdenum alloy coating on aluminum. This “Ce+Mo” processinvolves a chemical treatment of an aluminum alloy surface with Ce(NO₃)₃solution for several hours, followed by an electrochemical treatment(anodic polarization) in Na₂MoO₄ solution and finally a chemicaltreatment in CeCl₃ solution. The treatment process requires about sixhours to complete and it is difficult to control the chemical treatmentstep. In addition it is difficult to control the Ce—Mo composition andcoating distribution due to the chemical treatment process. A ceriumcoating can be electrodeposited on aluminum. However the conventionalelectrolytic method, which uses direct current (DC), involves a largeamount of hydrogen evolution due to the very negative reductionpotential of Ce³⁺ (−2.335 V vs Standard Hydrogen Electrode (SHE)). Theevolved hydrogen produces hydrogen embrittlement of the aluminum, stresscorrosion cracking (SCC) and corrosion fatigue of aluminum alloys.

[0033] Accordingly, a need has continued to exist for a method ofcontrolling the deleterious effects of hydrogen evolution inelectroplating processes and, in particular, for plating functionalchromium coatings from a Cr(III) plating bath that does not suffer fromthe disadvantages of current processes.

SUMMARY OF THE INVENTION

[0034] The problems of controlling evolution of hydrogen and its directand indirect effects on the properties of the electroplated coatings andthe adverse interaction of hydroxide ion with metal-bearing ions in theplating solution have now been alleviated by the process of the presentinvention wherein metal layers are deposited from a plating bath onto acathode substrate using a pulsed reverse current (PRC). The process isespecially applicable to electrodeposition of functional chromiumcoatings from a Cr(III) plating bath.

[0035] Accordingly, it is an object of the invention to provide a methodof controlling the evolution of hydrogen in electrodeposition of metalsat a cathode.

[0036] A further object is to control the pH in the vicinity of acathode at which metals are being deposited electrolytically.

[0037] A further object is to provide a functional chromium layer on asubstrate.

[0038] A further object is to provide a method for depositing afunctional chromium layer on a substrate using a Cr(III) plating bath.

[0039] A further object is to provide a method of depositing afunctional chromium layer using a pulsed reverse current waveform.

[0040] A further object is to provide a method for depositing gold on asubstrate.

[0041] A further object is to provide a method for depositing gold on asubstrate from a plating bath containing sulfite.

[0042] A further object is to provide a method for depositing Zn—Ni andZn—Sn alloy layers on a substrate.

[0043] A further object is to provide a method of depositing acerium-molybdenum anticorrosive layer on aluminum and aluminum alloys.

[0044] Further objects of the invention will become apparent from thedescription of the invention that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The sole FIGURE illustrates a pulsed reverse current waveform ofthe type used in the process of this invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

[0046] The method of the invention for controlling hydrogen evolutionand the deleterious effects thereof in electrodeposition of metals at acathodic substrate will be discussed in detail in connection with theapplication of the process to electrodeposition of a functional chromiummetal layer on a substrate using a plating bath containing trivalentchromium (Cr(III)) and using pulsed reverse current.

[0047] In electroplating from aqueous solutions, the electric currentflowing to the cathode is carried by the metal-bearing ions and thehydrogen ions. The hydrogen ions frequently carry a substantialfraction, often more than half, of the current. When the hydrogen ionsare discharged at the cathode to form hydrogen gas which is evolved, theconcentration of hydroxide (OH⁻) ions in the vicinity of the cathode isproportionately increased according to the well-known equilibriumreaction of water, and the pH increases. Thus the discharge of hydrogenions at the cathode effectively generates hydroxide ions at the cathode.The exact distance from the cathode surface to which the increasedhydroxide ion concentration, i.e., higher pH, exists will vary dependingon the conditions of the particular electroplating process such as totalcurrent, agitation of the bath and the like. In any case the increasedhydroxide concentration occurs in the vicinity of the cathode where thehydroxide ions can interact with any metal-bearing ions approaching thecathode if such ions are susceptible to reaction with hydroxide ions,the skilled practitioner will recognize that certain metal-bearing ionsdo not interact directly with hydroxide ions. For example, thedichromate ions (Cr₂O₇ ²⁻) that are commonly used in plating with Cr(VI)bear a negative charge, and do not react with hydroxide ions. Similarly,gold cyanide ions are not affected by hydroxide ions at theconcentrations usually produced in electroplating because of the greatstability of the gold-cyanide complex. However, other ions, e.g., Cr³⁺,Zn²⁺, and the like, are capable of reacting with hydroxide ions to forminsoluble precipitates at the cathode surface. When such ions are usedfor electroplating the deposition of the metal is inhibited by formationof reaction products with hydroxide ion, such as precipitates.

[0048] According to the process of this invention, the use of pulsedreverse current greatly reduces the evolution of hydrogen at the cathodeand thereby prevents the large increase in hydroxide ion concentrationin the vicinity of the cathode that occurs when DC current is used.

[0049] Chromium (III) ions are typical of those ions that can react withhydroxide ions produced in the vicinity of the cathode as a result ofthe discharge of the complementary hydrogen ions and their removal fromthe solution by evolution of hydrogen gas.

[0050] According to the invention, chromium plating of sufficientthickness to provide the properties of a functional chromium layer canbe efficiently and rapidly deposited on a substrate from a Cr(III)plating bath using a pulsed reverse current waveform. It will beunderstood by those skilled in the art that the pulsed reversed currentthrough the plating bath will be produced by a pulsed reverse voltageapplied to the electrodes of the electrolytic cell. Accordingly, theprocess of the invention may be described in terms of pulsed reversecurrent or pulsed reverse voltage. Hereinafter the process of theinvention will be discussed in terms of pulsed reverse current.

[0051] Plating of functional chromium layers from Cr(III) baths usingthe conventional direct-current procedure is not practical because thecurrent efficiency is low and the rate of electrodeposition isrelatively slow. Due to the rapid drop in current efficiency, thepractical limit for plating from a Cr(III) bath is about 2.5micrometers. Although the plating thickness increases rapidly when thecurrent is first applied, the deposition rate soon diminishes andeventually becomes very small. The reason for this behavior may be seenfrom a consideration of the chemistry involved in the electroplatingprocess.

[0052] The object to be plated is made the cathode of the electrolyticcell. At the cathode, chromium is deposited and hydrogen is evolved asrepresented in the following reactions, which include their standardpotentials relative to the standard hydrogen electrode (SHE):Cr³⁺ + 3e⁻ → Cr(φ⁰ = −0.74  V  vs  SHE)2H⁺ + 2e⁻ → H₂(φ⁰ = 0  V    vs  SHE)

[0053] Because the evolution of hydrogen at the cathode removes H⁺ ionsfrom the solution, the pH near the cathode surfaces increasesdramatically and chromic hydroxide (solubility product K_(sp)=5.4×10⁻³¹)precipitates in the high pH layer at the cathode. The sedimentation ofchromic hydroxide covers the cathode surface and its thickness increasesas the plating time and pH increase. This promotes an increase ofcathode polarization, a further decrease of chromium plating efficiency(i.e., an increase in the amount of hydrogen evolution reaction), andthe increase of impurities in the plating film. All these factors retardthe normal growth of crystals in the plating film, leading to theprevention of further plating of chromium. Finally, the evolution ofhydrogen continues as the only reaction. The precipitation of chromichydroxide at the cathode also results in surface cracks in the platedlayer and reduces the hardness and brightness of the chromium coating.

[0054] These deleterious effects are reduced by the use of a pulsedreverse current in the electrolytic cell.

[0055] A schematic diagram of a pulse reverse current (PRC) waveform isshown in FIG. 1. Each cycle of the waveform comprises a cathodic(forward) current pulse followed by an anodic (reverse) pulse and,optionally, a relaxation period. As will be understood from thefollowing discussion, a reverse pulse need not be present in every cycleto achieve the benefits of the invention, but it is preferred that therebe one reverse pulse in each cycle. The cathodic peak current isrepresented as i₁, and the cathodic on-time is t₁. The anodic current isrepresented as i₂, and the anodic on-time is t₂. The relaxation time ist_(o). The sum of the cathodic on-time, anodic on-time, and relaxationtime is the period of the pulse (T=t₀+t₁+t₂), and the inverse of theperiod is defined as the frequency of the pulsed current. The ratio ofthe cathodic on-time to the period (t₁/T) is the cathodic duty cycle(D₁), and the ratio of the anodic on time to the period (t₂/T) is theanodic duty cycle (D₂). The current density during the cathodic on-timeand anodic on-time is known as the cathodic peak pulse current densityand anodic peak pulse current density, respectively. The average currentdensity (i_(ave)) is the average cathodic current density (D₁i₁) minusthe average anodic current density (D₂i₂).

[0056] Once the average current density (i_(ave)), pulse frequency (f),cathodic duty cycle (D₁), anodic duty cycle (D₂), and the cathodic toanodic charge ratio (Q₁/Q₂) are given, the cathodic and anodic on-timeand relaxation time (t₁, t₂, and t₀) and cathodic and anodic peakcurrent density (i₁ and i₂) are determined from the following equations:$T = \frac{1}{f}$ $D_{1} = \frac{t_{1}}{T}$ $D_{2} = \frac{t_{2}}{T}$$\frac{Q_{1}}{Q_{2}} = \frac{i_{1}t_{1}}{i_{2}t_{2}}$i_(ave) = i₁D₁ − i₂D₂ T = t_(o) + t₁ + t₂

[0057] Another condition is:

D ₁ +D ₂≦1

[0058] It should be noted that the cathodic on-time, anodic on-time, andrelaxation time, and the cathodic and anodic peak pulse current densityare additional parameters available to control the electroplatingprocess compared to conventionally used DC plating wherein the cathodiccurrent flows for the entire duration of the plating process.Furthermore, the higher peak current in the forward cathodic phase ofthe pulsed reverse current cycle produces a finer grained deposit thanthat produced by DC of the same average current, thereby yielding aharder chromium layer. The reverse current portion of the cycle consumesnascent hydrogen, thereby keeping the local pH relatively low andprevents the formation of chromium hydroxide which adversely affects thehardness of the deposit. The consumption of nascent hydrogen also helpsto avoid hydrogen embrittlement of the deposit. The relaxation period ofthe current cycle contributes to enhanced current efficiency and platingrate by allowing time for the chromium ions to diffuse to the cathodesurface, thereby increasing the local concentration of chromium ionsduring the forward, plating portion of the cycle.

[0059] For chromium plating from a Cr(III) bath, a cathodic pulse isused having either a long duty cycle or a large pulse current to depositchromium, followed by an anodic pulse with either a short duty cycle ora small pulse current to convert the nascent hydrogen gas formed duringthe cathodic cycle to H⁺, and a relaxation period to allow the Cr(III)ions to diffuse to the cathode surface and be available for subsequentdeposition. During the cathodic portion of the pulse, chromium isdeposited and hydrogen is evolved, analogous to the conventionalchromium plating. During the anodic portion of the pulse the nascenthydrogen is consumed according to the following reaction:

H₂→2H⁺+2e ⁻

[0060] In this manner, a low pH is maintained and chromic hydroxideprecipitation is avoided. By properly adjusting the anodic and cathodicpeak currents, the anodic and cathodic duty cycles, and the PRCfrequency, nascent hydrogen can be consumed. Furthermore, the parameterscan also be adjusted to provide that the net plating rate from theCr(III) plating bath is equivalent to that achieved in current practiceusing Cr(VI) baths.

[0061] In DC plating, chemical additives are used to produce the desiredproperties of metal coatings. The additives influence a number ofproperties of deposits, including 1) brightness, 2) hardness, 3)corrosion resistance, and 4) mechanical characteristics, such asstrength and ductility. In PRC plating the grain size and coatingmorphology can be adjusted by modifying the PRC wave forms. Therefore,desired coating properties can be obtained with PRC plating from anadditive-free plating bath. If the bath is additive-free, the usedsolution can be replenished and recycled to the plating tank withoutcontaminated drag-in and zero discharge will be obtained. This willeliminate the disposal of toxic chromium compounds and reduce theenvironmental impact.

[0062] The trivalent chromium bath used for functional chromium platingaccording to the process of the invention may contain any salt oftrivalent chromium that is sufficiently soluble to achieve a practicalconcentration of trivalent chromium ions in the solution. Suitable bathformulations containing trivalent chromium are disclosed in U.S. Pat.No. 5,415,763, the entire disclosure of which is incorporated herein byreference. Preferred salts are CrCl₃ and KCr(SO₄)₂. The bath may containone or more suitable salts. The amount of trivalent chromium salt in theplating bath may range from about 0.6 g/L to about 40 g/L, preferablyfrom about 22 g/L to about 28 g/L, calculated as the weight of trivalentchromium ions in the solution, that is from about 0.01 moles/liter toabout 0.77 moles/liter, preferably from about 0.42 moles/L to about 0.54moles/L of trivalent chromium. The pH of the plating bath may range fromabout 1.5 to about 3.5, preferably from about 1.9 to 2.6.

[0063] The plating bath may contain other materials to assist in theplating process and improve the properties of the plated deposit.Preferred bath compositions are those given in the example below.

[0064] The frequency of the pulsed reverse current may be from about 5Hz to about 700 Hz, preferably from about 10 Hz to about 200 Hz. Theduty cycle of the forward pulses may range from about 50% to about 90%,preferably from about 80% to about 90%. The duty cycle of the reversepulses may range from about 5% to about 30%, preferably from about 5% toabout 15%. The cathodic to anodic charge ratio, i.e., the ratio ofcurrent carried by the forward pulses to that carried by the reversepulses may range from about 5: to about 80:1. The reverse pulses may beinterposed between some or all of the forward pulses or a plurality ofreverse pulses may be interposed between some or all of the forwardpulses. It is preferred that the forward and reverse pulses alternate sothat one reverse pulse is interposed between each pair of forwardpulses, providing a pulse cycle as shown in FIG. 1. It will also berecognized by those skilled in the art that the pulses need not have thesquare waveform shown in FIG. 1. Any waveform that provides a forwardpulse of current and a reverse pulse of current is suitable. Thus thewave forms may be, for example, square, trapezoidal sinusoidal, or evenirregular or the like, so long as they provide for a forward cathodicduty cycle and a reverse anodic duty cycle. An asymmetrical sine wavewould also be a suitable wave form. The actual shape of the waveformused in a particular application will be determined by practicalconsiderations of electrical current supply equipment.

[0065] The invention will be illustrated by the following example, whichis intended to be illustrative and not limiting.

EXAMPLE

[0066] This example illustrates deposition of a functional chromiumlayer on a steel substrate.

[0067] A steel rod 1.2 cm in diameter by 28 cm in length, of the typegenerally used in automotive shock absorbers, was measured with amicrometer and then prepared for electroplating by a conventionalthree-step treatment comprising an alkaline soak cleaning,electrocleaning, and an acid etch. Between each step of the preparationprocess the rod was thoroughly rinsed with water.

[0068] The rod was then mounted in a laboratory-scale electrolytic cellholding about 3 liters of electrolyte. The rod was configured as thecathode and the anode was an inert electrode.

[0069] Experiments were conducted using two different plating bathsolutions. The first plating bath (bath A) was an aqueous solutioncontaining the following ingredients in the listed concentrations.CrCl₃.6H₂O 125 g/L Cr(SO₄)₂.12H₂O  25 g/L NH₄NH₂SO₃ 178 g/L NH₄Cl  80g/L H₃BO₃  31 g/L HCOOH  60 mL/L

[0070] The pH of plating bath A was adjusted with potassium hydroxide toa value of 2.5. The specific gravity of the solution was 1.2. In bath B,the amount of formic acid was decreased to 30 mL/L, and the bath wasused at its unadjusted pH of 2.0, as prepared. The electroplating wasconducted at a temperature in the range of 20-60° C. to simulatecommercial plating conditions in which the bath temperature is notclosely controlled.

[0071] Test samples were plated using direct current and pulse reversecurrent (PRC) at various frequencies, average currents (i_(ave)),cathodic and anodic duty cycles (D_(c) and D_(a)) and charge transferratios (Q_(c)/Q_(a)).

[0072] After plating the rods were rinsed and dried and their diameterswere measured with a micrometer to determine the thickness of theplating. The results are summarized in Table 1 below. TABLE 1 PlatingPlating Plating Current I_(ave) f D_(c) D_(a) Thickness Time RateEfficiency Test (A) (Hz) (%) (%) Q_(c)/Q_(a) Bath (μm) (min) (μm/min)(%) 1 30 DC A 15.7 30 0.76 18.5 2 30 100 80 5 10:1 A 40.6 30 1.35 33 330 100 90 8 10:1 A 40.6 30 1.35 33 4 30 100 90 5 20:1 A 43.2 30 1.44 355 30 100 50 25 10:1 A 25.4 20 1.27 31 6 30 100 50 5 10:1 A 25.4 25 1.0124.6 7 30  10 20 10 10:1 A 25.4 25 1.01 24.6 8 30 100 20 5 10:1 A 25.425 1.01 24.6 9 30 500 50 10 10:1 A 25.4 25 1.01 24.6 10 40  10 80 8 20:1A 55.9 30 1.86 34 11 40 100 50 10 10:1 A 25.4 20 1.27 23.1 12 30 100 805 40:1 B 18.5 10 1.85 34 13 94 100 80 5 40:1 B 35 10 3.5 20.5

[0073] For comparison purposes, a similar rod plated with a functionalchromium layer from a conventional Cr(VI) bath exhibited a platingthickness of 8.9 -12.7 μm, at a plating time of 10 min, a plating rateof 0.76-1.27 μm/min, and a current efficiency of 24%.

[0074] The best results with the process of the invention using bath Awere those of tests 2 and 3 which achieved a plating rate of 1.35 μm/minand a current efficiency of 33%. The best overall result was achieved intest 12 wherein the plating rate was 1.85 μm/min and the currentefficiency was 34%. This sample also had the best hardness and lowestporosity of the test samples. In test 13 a higher average currentproduced a greater plating rate. However, the coating had a greaterporosity and lower hardness that the sample of test 12.

[0075] The test results also show that the process of the invention issuperior to the conventional process for electroplating decorativechromium layers from a Cr(III) bath using direct current wherein thedeposition of chromium metal essentially ceases after a layer about 2.5μm thick has been plated because of the precipitation of a layer ofchromium hydroxide.

[0076] The data establish that functional chromium layers can bedeposited from a Cr(III) bath with plating rate and current efficiencycomparable to those achieved with a conventional hexavalent chromiumbath. Furthermore, the plating rate data show that pulsed reversecurrent can achieve a plating rate substantially greater than thatobtained using direct current. The current efficiency of plating withpulsed reverse current is significantly greater than that of DC platingand comparable with that of conventional chromium plating. The hardnessof the chromium layer deposited using pulsed reverse current underoptimum conditions was equivalent to that of chromium coatings preparedby conventional plating from a hexavalent chromium bath.

[0077] The invention having now been fully described, it should beunderstood that it may be embodied in other specific forms or variationswithout departing from its spirit or essential characteristics.Accordingly, the embodiments described above are to be considered in allrespects as illustrative and not restrictive, the scope of the inventionbeing indicated by the appended claims rather than the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are intended to be embraced therein.

We claim:
 1. A method for electrolytic deposition of metals on a cathodesubstrate comprising immersing an electrically conductive anode and anelectrically conductive cathode in an aqueous plating bath containingmetal-bearing ions, hydrogen ions, and hydroxide ions, saidmetal-bearing ions being capable of migrating to said cathode and beingdischarged at said cathode and depositing metal thereon, said hydrogenions being capable of migrating to said cathode and being discharged atsaid cathode to form hydrogen gas, whereby the concentration of saidhydroxide ions in the vicinity of said cathode is increased, saidmetal-bearing ions being capable of reacting with said hydroxide ions inthe vicinity of said cathode whereby deposition of said metal on saidcathode is inhibited; passing an electric current from said anode tosaid cathode through said plating bath, whereby said metal-bearing ionscarry a first fraction of said current by migrating to said cathode andbeing discharged at said cathode, and said hydrogen ions carry a secondfraction of said current by migrating to said cathode and beingdischarged at said cathode, said first fraction and said second fractiontogether constituting said electric current, wherein said electriccurrent is a pulsed reverse current having forward pulses and reversepulses.
 2. The method of claim 1 wherein said pulsed reverse current hasa frequency of about 5 Hz to about 700 Hz.
 3. The method of claim 1wherein said pulsed reverse current has a frequency of about 10 Hz toabout 200 Hz.
 4. The method of claim 1 wherein said forward pulses ofsaid pulsed reverse current have a duty cycle of from about 50% to about90%.
 5. The method of claim 6 wherein said forward pulses of said pulsedreverse current have a duty cycle of from about 80% to about 90%.
 6. Themethod of claim 1 wherein said reverse pulses of said pulsed reversecurrent have a duty cycle of from about 5% to about 30%.
 7. The methodof claim 1 wherein said reverse pulses of said pulsed reverse currenthave a duty cycle of from about 5% to about 15%.
 8. The method of claim1 wherein a reverse pulse is interposed between at least some of saidforward pulses.
 9. The method of claim 1 wherein a reverse pulse isinterposed between each pair of forward pulses.
 10. The method of claim1 wherein the ratio of electric charge carried by said forward pulses toelectric charge carried by said reverse pulses is from about 5:1 toabout 80:1.
 11. The method of claim 1 wherein a relaxation period of nocurrent flow is interposed between a reverse pulse and a followingforward pulse.
 12. The method of claim 1 wherein said metal-bearing ionscontain trivalent chromium.
 13. The method of claim 1 wherein saidmetal-bearing ions contain gold.
 14. The method of claim 1 wherein saidmetal-bearing ions comprise ions containing zinc and ions containingnickel.
 15. The method of claim 1 wherein said metal-bearing ionscomprise ions containing zinc and ions containing tin.
 16. The method ofclaim 1 wherein said metal-bearing ions comprise ions containing ceriumand ions containing cobalt.
 17. In a method for electrolytic depositionof metals on a cathode substrate by immersing an anode and a cathode inan aqueous plating bath containing metal-bearing ions and passing anelectric current between said anode and said cathode, wherein saidmetal-bearing ions are capable of migrating to said cathode and beingdischarged at said cathode, and are capable of reacting with hydroxideions generated in the vicinity of said cathode by evolution of hydrogenat said cathode, the improvement comprising controlling excessiveevolution of hydrogen at said cathode by using as said electric currenta pulsed reverse current.
 18. A method of depositing a layer of chromiummetal on a substrate comprising providing an aqueous plating bathcontaining a water-soluble salt of trivalent chromium; immersing in saidplating bath a cathode comprising an electrically conducting substrateand an anode; passing an electric current between said anode and saidcathode wherein said current is a pulsed reverse current having afrequency of from about 5 Hz to about 700 Hz, said pulsed currentcomprising forward pulses having a duty cycle of from about 25% to about95%, and reverse pulses having a duty cycle of from about 50% to about5%, said reverse pulses being interposed between at least some of saidforward pulses.
 19. The method of claim 18 wherein said bath containsone or more water-soluble salts of trivalent chromium and the totalconcentration of trivalent chromium in said bath is from about 3 g/L toabout 200 g/L.
 20. The method of claim 18 wherein said bath contains oneor more water-soluble salts of trivalent chromium and the totalconcentration of trivalent chromium in said bath is from about 3 g/L toabout 200 g/L.
 21. The method of claim 18 wherein said totalconcentration of trivalent chromium salt in said bath is from about 110g/L to about 140 g/L.
 22. The method of claim 18 wherein said platingbath has a pH of from about 1.5 to about 3.5.
 23. The method of claim 22wherein said plating bath has a pH of from about 1.9 to about 2.6. 24.The method of claim 23 wherein said plating bath has a pH of about 2.5.25. The method of claim 18 wherein said water-soluble salt of trivalentchromium is selected from the group consisting of CrCl₃ and KCr(SO₄)₂.26. The method of claim 18 wherein said pulsed current has a frequencyof about 10 Hz to about 200 Hz.
 27. The method of claim 18 wherein saidforward pulses of said pulsed current have a duty cycle of from about50% to about 90%.
 28. The method of claim 18 wherein said forward pulsesof said pulsed current have a duty cycle of from about 80% to about 90%.29. The method of claim 18 wherein said reverse pulses of said pulsedreverse current have a duty cycle of from about 30% to about 5%.
 30. Themethod of claim 18 wherein said reverse pulses of said pulsed reversecurrent have a duty cycle of from about 15% to about 5%.
 31. The methodof claim 18 wherein the ratio of electric charge carried by said forwardpulses to electric charge carried by said reverse pulses is from about5:1 to about 80:1.
 32. The method of claim 18 wherein a reverse pulsesis interposed between at least some of said forward pulses.
 33. Themethod of claim 18 wherein a reverse pulse is interposed between eachpair of forward pulses.
 34. The method of claim 18 wherein a relaxationperiod of no current flow is interposed between a reverse pulse and afollowing forward pulse.